This invention relates to mass rate of flow meters of the angular momentum type having a swirl generator for imparting angular momentum to a measured fluid stream and a restrained reaction turbine for removing the imparted angular momentum. More particularly, the invention relates to such flowmeters having an improved construction for preventing leakage and optimizing fluid flow at low flow rates.
A mass rate of flow meter of the angular momentum type is disclosed by Hildebrand et al in U.S. Pat. No. 4,056,976, issued Nov. 8, 1977, titled "Mass Rate of Flow Meter" and assigned to the same assignee as the present invention. In this prior art flowmeter, a swirl generator includes radially extending skewed vanes that impart angular momentum to the fluid passing between the vanes. Swirling fluid from the swirl generator passes through multiple tubes in an unrestrained rotor located downstream. Fluid leaving the rotor has the same angular velocity as the rotor. The restrained reaction turbine also includes multiple tubes that form longitudinal flow channels. As swirling fluid leaves the rotor, the restrained reaction turbine removes the angular momentum from fluid flowing through the longitudinal channels. The torque it experiences in doing so is balanced by a restraining means including a biased spring that permits measurement of flow rate by methods not material to the present invention.
U.S. Pat. No. 3,538,767, issued Nov. 10, 1970, titled "Flowmeter Fluid Drive", and assigned to the same assignee as the present invention, discloses another type of flowmeter that incorporates a torque motor for restraining the turbine. This flowmeter includes a cylindrical conduit within the flowmeter casing for conducting fluid to the swirl generator. The conduit comprises longitudinally extending resilient fingers that maintain a substantially cylindrical shape of the conduit at low flow rates in order that the conduit directs as much incoming fluid as possible to the swirl generator. At high flow rates, the same resilient fingers deflect outwardly in response to fluid pressure to control the angular momentum imparted to the fluid.
Thus the total fluid flow downstream from the swirl generator is a mixture of flows that have gone through the vanes on the swirl generator or bypassed them under the control of the conduit. This control prevents the swirl velocity of the fluid entering the reaction turbine from becoming excessive at high flow rates.
In flowmeters of the type described in the foregoing U.S. Pat. No. 4,056,976, timing circuits sense start and stop pulses induced in a pair of coils and use these pulses to determine rotor speed and to determine deflection of the restrained reaction turbine. The amplitude and width of these pulses vary with rotor speed. In the unrestrained rotor flowmeters, however, the rotor can have a wider range of angular velocities (e.g., from 1 to 6 revolutions per second). Thus, the timing circuits must either include circuitry for compensating these variations or operate with inaccuracies.
At low flow rates, it has been found that even compensated flowmeters of this type tend to have non-linear errors that are not readily compensated. Several sources of these inaccuracies have been found. One source is fluid leakage between the individual resilient fingers of the cylindrical conduit. This leakage limits the percentage of fluid flow affected by the swirl generator at low flow rates. As a result, the angular velocity of the rotor decreases as the flow rate decreases. Moreover, the angular velocity can reach a level at which accurate measurements of the start and stop pulse timing become difficult to achieve.